A Universal Mobile Telecommunication System (UMTS) is a broadband, packet-based system for the transmission of text, digitized voice, video and multi-media. It is a highly subscribed to standard for third generation and is generally based on Wideband Coded Division Multiple Access (W-CDMA).
In a UMTS network, a Radio Resource Control (RRC) part of the protocol stack is responsible for the assignment, configuration and release of radio resources between the UE and the UTRAN. This RRC protocol is described in detail in the 3GPP TS 25.331 specifications. Two basic modes that the UE can be in are defined as “idle mode” and “UTRA connected mode”. UTRA stands for UMTS Terrestrial Radio Access. In idle mode, the UE is required to request an RRC connection whenever it wants to send any user data or in response to a page whenever the UTRAN or the Serving GPRS Support Node (SGSN) pages it to receive data from an external data network such as a push server. Idle and Connected mode behaviors are described in details in 3GPP specifications TS 25.304 and TS 25.331.
When in a UTRA RRC connected mode, the device can be in one of four states. These are:
CELL_DCH: A dedicated channel is allocated to the UE in uplink and downlink in this state to exchange data. The UE must perform actions as outlined in 3GPP 25.331.
CELL_FACH: no dedicated channel is allocated to the user equipment in this state. Instead, common channels are used to exchange a small amount of bursty data. The UE must perform actions as outlined in 3GPP 25.331 which includes the cell selection process as defined in 3GPP TS 25.304.
CELL_PCH: the UE uses Discontinuous Reception (DRX) to monitor broadcast messages and pages via a Paging Indicator Channel (PICH). No uplink activity is possible. The UE must perform actions as outlined in 3GPP 25.331 which includes the cell selection process as defined in 3GPP TS 25.304. The UE must perform the CELL UPDATE procedure after cell reselection.
URA_PCH: the UE uses Discontinuous Reception (DRX) to monitor broadcast messages and pages via a Paging Indicator Channel (PICH). No uplink activity is possible. The UE must perform actions as outlined in 3GPP 25.331 including the cell selection process as defined in 3GPP TS 25.304. This state is similar to CELL_PCH, except that the URA UPDATE procedure is only triggered via UTRAN Registration Area (URA) reselection.
The transition from an idle to the connected mode and vice-versa is controlled by the UTRAN. When an idle mode UE requests an RRC connection, the network decides whether to move the UE to the CELL_DCH or CELL_FACH state. When the UE is in an RRC connected mode, again it is the network that decides when to release the RRC connection. The network may also move the UE from one RRC state to another prior to releasing the connection. The state transitions are typically triggered by data activity or inactivity between the UE and the network. Since the network may not know when the UE has completed data exchange, it typically keeps the RRC connection for some time in anticipation of more data to/from the UE. This is typically done to reduce the latency of call setup and radio bearer setup. The RRC connection release message can only be sent by the UTRAN. This message releases the signal link connection and all radio bearers between the UE and the UTRAN.
The problem with the above is that even if an application on the UE has completed its data transaction and is not expecting to any further data exchange, it still waits for the network to move it to the correct state. The network may not be even aware of the fact that the application on the UE has completed its data exchange. For example, an application on the UE may use its own acknowledgement-based protocol to exchange data with its application server which is connected to the UMTS core network. Examples are applications that run over UDP/IP implementing their own guaranteed delivery. In such a case, the UE knows whether the application server has sent or received all the data packets or not and is in a better position to determine if any further data exchange is to take place and hence decide when to terminate the RRC connection. Since the UTRAN controls when the RRC connected state is changed to a different, less battery-intensive state or into an idle mode, and the fact that UTRAN is not aware of the status of data delivery between the UE and external server, the UE is forced to stay in a higher data rate and intensive battery state than the required state or mode, thereby draining battery life and wasting network resources.